Compressors, for example refrigerator compressors, are conventionally driven by rotary electric motors. However, even in their most efficient form, there are significant losses associated with the crank system that converts rotary motion to linear reciprocating motion. Alternatively a rotary compressor which does not require a crank can be used but again there are high centripetal loads, leading to significant frictional losses. A Linear compressor driven by a linear motor would not have these losses, and can be designed with a bearing load low enough to allow the use of aerostatic, gas bearings as disclosed in U.S. Pat. No. 5,525,845.
Linear reciprocating motors obviate the need for crank mechanisms which characterise compressors powered by rotating electric motors and which produce high side forces requiring oil lubrication. Such a motor is described in U.S. Pat. No. 4,602,174. U.S. Pat. No. 4,602,174 discloses a linear motor design that is extremely efficient in terms of both reciprocating mass and electrical efficiency. This design has been used very successfully in motors and alternators that utilise the Stirling cycle. It has also been used as the motor for linear compressors. However, in the case of compressors designed for household refrigerators the design in U.S. Pat. No. 4,602,174 is somewhat larger and more costly than is desirable for this market.
The piston of a free piston compressor oscillates in conjunction with a spring as a resonant system and there are no inherent limits to the amplitude of oscillation except for collision with a stationary part, typically part of the cylinder head assembly. The piston will take up an average position and amplitude that depend on gas forces and input electrical power. Therefore for any given input electrical power, as either evaporating or condensing pressure reduces, the amplitude of oscillation increases until collision occurs. It is therefore necessary to limit the power as a function of these pressures.
It is desirable for maximum efficiency to operate free piston refrigeration compressors at the natural frequency of the mechanical system. This frequency is determined by the spring constant and mass of the mechanical system and also by the elasticity coefficient of the gas. In the case of refrigeration, the elasticity coefficient of the gas increases with both evaporating and condensing pressures. Consequently the natural frequency also increases. Therefore for best operation the frequency of the electrical ten powering the compressor needs to vary to match the mechanical system frequency as it varies with operating conditions.
Methods of synchronising the electrical voltage applied to the compressor motor windings with the mechanical system frequency are well known. For a permanent magnet motor used in a free piston compressor, a back electromotive force (back EMF) is induced in the motor windings proportional to the piston velocity as shown in FIG. 8a The equivalent circuit of the motor is shown in FIG. 8b. An alternating voltage (V) is applied in synchronism with the alternating EMF (αν) in order to power the compressor. U.S. Pat. No. 4,320,448 (Okuda et al.) discloses a method whereby the timing of the applied voltage is determined by detecting the zero crossings of the motor back EMF. The application of voltage to the motor winding is controlled such that the current is zero, at the time at which the EMF intersects with the zero level to allow back EMF zero crossing detection.
Various methods have been used to limit oscillation amplitude including secondary gas spring, piston position detection, piston position calculation based on current and applied voltage (U.S. Pat. No. 5,496,153) measuring ambient and/or evaporating temperature (U.S. Pat. No. 4,179,899, U.S. Pat. No. 4,283,920). Each of these methods requires the cost of additional sensors or has some performance limitation.